Polycyclic aromatic hydrocarbon: Map

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Polycyclic aromatic
hydrocarbons (PAHs) are chemical compounds that consist of fused
aromaticrings and do not contain heteroatoms or carry substituents. PAHs occur in oil, coal, and tar deposits, and are produced as byproducts of fuel
burning (whether fossil fuel or biomass). As a pollutant, they are
of concern because some compounds have been identified as carcinogenic, mutagenic,
and teratogenic. PAHs are also found in
foods. Studies have shown that most food intake of PAHs comes from
cereals, oils and fats. Smaller intakes come from vegetables and
cooked meats.

Occurrence and pollution

Polycyclic aromatic hydrocarbons are lipophilic, meaning they mix more easily with oil
than water. The larger compounds are less water-soluble and less
volatile (i.e., less prone to
evaporate). Because of these properties, PAHs in the environment
are found primarily in soil, sediment and oily
substances, as opposed to in water or air. However, they are also a
component of concern in particulate
matter suspended in air.

Natural crude oil and coal deposits contain significant amounts of
PAHs, arising from chemical conversion of natural product
molecules, such as steroids, to aromatic hydrocarbons. They are
also found in processed fossil fuels, tar and
various edible oils.

PAHs are one of the most widespread organic pollutants. In addition
to their presence in fossil fuels they are also formed by
incomplete combustion of carbon-containing fuels such as wood, coal, diesel, fat, tobacco, or incense.
Different types of combustion yield different distributions of PAHs
in both relative amounts of individual PAHs and in which isomers are produced. Thus coal burning produces a
different mixture than motor-fuel combustion or a forest fire,
making the compounds potentially useful as markers.

Hydrocarbon emissions from fossil fuel-burning engines are
regulated in developed countries.

Human health

PAHs toxicity is very structurally dependent, with isomers (PAHs
with the same formula and number of rings) varying from being
nontoxic to being extremely toxic. Thus, highly carcinogenic PAHs
may be small or large. One PAH compound, benzo[a]pyrene, is notable for being the first
chemical carcinogen to be discovered (and is one of many
carcinogens found in cigarette
smoke). The EPA has classified seven PAH compounds as probable
human carcinogens: benz[a]anthracene, benzo[a]pyrene,
benzo[b]fluoranthene, benzo[k]fluoranthene, chrysene,
dibenz[a,h]anthracene, and indeno[1,2,3-cd]pyrene.

Chemistry

The simplest PAHs, as defined by the International Union on Pure
and Applied Chemistry (IUPAC) {G.P Moss, IUPAC nomenclature for
fused-ring systems), are phenanthrene
and anthracene, which both contain three
fused aromatic rings. Smaller molecules, such as benzene, are not PAHs.

PAHs may contain four-, five-, six- or seven-member rings, but
those with five or six are most common. PAHs composed only of
six-membered rings are called alternant PAHs.
Certain alternant PAHs are called "benzenoid" PAHs. The name comes
from benzene, an aromatic hydrocarbon with a single,
six-membered ring. These can be benzene rings interconnected with
each other by single carbon-carbon bonds and with no rings
remaining that do not contain a complete benzene ring.

The set of alternant PAHs is closely related to a set of
mathematical entities called polyhex, which are planar figures
composed by conjoining regular hexagons of
identical size.

PAHs containing up to six fused aromatic rings are often known as
"small" PAHs and those containing more than six aromatic rings are
called "large" PAHs. Due to the availability of samples of the
various small PAHs, the bulk of research on PAHs has been of those
of up to six rings. The biological activity and occurrence of the
large PAHs does appear to be a continuation of the small PAHs. They
are found as combustion products, but at lower levels than the
small PAHs due to the kinetic limitation of their production
through addition of successive rings. Additionally, with many more
isomers possible for larger PAHs, the occurrence of specific
structures is much smaller.

PAHs possess very characteristic UV absorbance spectra.
These often possess many absorbance bands and are unique for each
ring structure. Thus, for a set of isomers,
each isomer has a different UV absorbance spectrum than the others.
This is particularly useful in the identification of PAHs. Most
PAHs are also fluorescent, emitting
characteristic wavelengths of light when they are excited (when the
molecules absorb light). The extended pi-electron electronic
structures of PAHs lead to these spectra, as well as to certain
large PAHs also exhibiting semi-conducting and other behaviors.

Naphthalene (C10H8
constituent of mothballs), consisting of
two coplanar six-membered rings sharing an edge, is another
aromatic hydrocarbon. By formal convention, it is not a true PAH,
though is referred to as a bicyclic aromatic hydrocarbon.

Aqueous solubility decreases approximately one order of magnitude
for each additional ring.

PAH compounds

The U.S. EPA has designated 16 PAH compounds as priority
pollutants. They are naphthalene, acenaphthylene, acenaphthene,
fluorene, phenanthrene, anthracene, fluoranthene, pyrene,
benz[a]anthracene, chrysene, benzo[b]fluoranthene,
benzo[k]fluoranthene, benzo[a]pyrene, dibenz[a,h]anthracene,
benzo[g,h,i]perylene, and indeno[1,2,3-cd]pyrene. This list of the
16 EPA priority PAHs is often targeted for measurement in
environmental samples.

Aromaticity

Although PAHs clearly are aromatic compounds, the degree of
aromaticity can be different for each
ring segment. According to Clar's rule (formulated
by Erich Clar in 1964) for PAHs the resonance structure with the most
disjoint aromatic п-sextets—i.e. benzene-like moieties—is the most
important for the characterization of the properties.

For example, in phenanthrene the Clar
structure 1A has two sextets at the extremities,
while resonance structure 1B has just one central sextet. Therefore
in this molecule the outer rings are firmly aromatic while its
central ring is less aromatic and therefore more reactive. In
contrast, in anthracene2 the number of sextets is just one and
aromaticity spreads out. This difference in number of sextets is
reflected the UV absorbance spectra of these two isomers. Phenanthrene has a highest wavelength
absorbance around 290 nm, while anthracene has highest wavelength bands around
380 nm. Three Clar structures with two sextets are present in
chrysene (4) and by
superposition the aromaticity in the outer ring is larger than in
the inner rings.

Origins of life

In January 2004 (at the 203rd Meeting of the American Astronomical
Society), it was reported that a team led by A. Witt of the University of
Toledo, Ohio studied ultraviolet light emitted by the Red Rectangle nebula and found the
spectral signatures of anthracene and
pyrene (no other such complex molecules had
ever before been found in space). This discovery was
considered confirmation of a hypothesis that as nebulae of the same
type as the Red Rectangle approach the ends of their lives,
convection currents cause carbon and hydrogen in the nebulae's core
to get caught in stellar winds, and radiate outward. As they cool,
the atoms supposedly bond to each other in various ways and
eventually form particles of a million or more atoms. Witt and his
team inferred (as cited in Battersby, 2004) that since they
discovered PAHs—which may have been vital in the formation of early
life on Earth—in a nebula, by necessity they must originate in
nebulae.

References

http://ec.europa.eu/food/fs/sc/scf/out154_en.pdf

http://www.atsdr.cdc.gov/tfacts69.html#bookmark02

For example, EPA regulations for small engines are at 40 CFR
§90.103; see emission standard for more
information.